Computation-based regulation of excitonic effects in donor-acceptor covalent organic frameworks for enhanced photocatalysis

The strong excitonic effects widely exist in polymer-semiconductors and the large exciton binding energy (Eb) seriously limits their photocatalysis. Herein, density functional theory (DFT) calculations are conducted to assess band alignment and charge transfer feature of potential donor-acceptor (D-A) covalent organic frameworks (COFs), using 1,3,5-tris(4-aminophenyl)triazine (TAPT) or 1,3,5-tris(4-aminophenyl)benzene (TAPB) as acceptors and tereph-thaldehydes functionalized diverse groups as donors. Given the discernable D-A interaction strengths in the D-A pairs, their Eb can be systematically regulated with minimum Eb in TAPT-OMe. Guided by these results, the corresponding D-A COFs are synthesized, where TAPT-OMe-COF possesses the best activity in photocatalytic H2 production and the activity trend of other COFs is associated with that of calculated Eb for the D-A pairs. In addition, further alkyne cycloaddition for the imine linkage in the COFs greatly improves the stability and the resulting TAPT-OMe-alkyne-COF with a substantially smaller Eb exhibits ~20 times higher activity than the parent COF.

This paper reports the impact of exciton binding energy of donor-acceptor covalent organic frameworks in photocatalytic H<sub>2</sub> productionon. The authors provide a detailed description on how the exciton binding energy can be regulated to increase the photocatalytic efficiency. The theoretical results are compared with experiments demonstrating an excellent correlation. In addition, the issue of process stability was addressed, where an interesting and simple molecular modification in the acceptor component was successfully employed. Interestingly, the molecular modification also led to a significant increase in the H<sub>2</sub> production rate. The possibility of using the theory to guide the synthesis process of the materials completely developed in this work proved to be very efficient. The manuscript is well written, easy to follow, and it does improve our understanding about the correlation between exciton binding energy and photocatalytic H<sub>2</sub> productionon, in my judgment. I enjoyed reading the paper and recommend it for publication. I have a few comments (see below) that the authors should take into account.
Minor remark: 1. In my opinion, the first paragraph of the manuscript lacks mention of the state of the art production of H<sub>2</sub>. Likewise, at the end of the manuscript it is necessary to correlate the results obtained with the state of the art.
2. In the same context as my first comment, is there any advantage of the method used in this report to produce H<sub>2</sub> over other methods? For example, with regards to complexity.
3. In the introduction, it is necessary to clearly mention theoretical works (DFT) in the area of organic solar cells that also use this strategy of weakening the exciton binding energy of molecules to optimize the photovoltaic process (https://doi.org/10.1021/acs.jpcc.8b12261, https://doi.org/10.1021/acs.jpcc.8b07197, https://doi.org/10.1039/C9TC03563J). Furthermore, in the introduction it is mentioned that "...D-A molecular junctions inspired by solar cells have received intensive attention due to easy construction and structural diversity.'' If I'm not mistaken, the three references cited in this sentence do not correspond to the statement. 4. Some figures (1-4) are not in good resolution and should be improved. 5-It would be interesting to specify the functional and basis of DFT calculations also in the supplementary information.
6. There are two references 46, this should be fixed with renumbering. 7. Does the dihedral angle between the D and A components of the studied molecules have any correlation with the exciton binding energy?
Response to Reviewer 1: General Comment: "Qian, Han, Zhang, Yang, Zhang and Jiang tried to clarify the relationship between structure and exciton binding energy in donor-acceptor based covalent organic framework (COF). In the present paper, the authors focused on the COF based on TAPB/TAPT as acceptor group and terephthaldehyde derivatives as donor group. The energy levels of the groups have been adjusted by the aspects derived from theoretical calculations, besides, the nature of charge transfer by several computational approach. The trends were also captured experimentally, suggesting the theoretical predictions as successful for fine tuning of photon (light) energy harvesting. Modification of the two COFs further improved their stability and H2 production rate: the clear feedbacks from the computation. "Computation-Guided Regulation": apparently the highlight of the present work as the authors are claiming in the title. The reviewer agrees the computational protocols are successful for the design of the COF system, however feels simultaneously the claims are too much after reading this manuscript. Most of all the readers may be disappointed by the story in contrast to the overstated title. The examined pairs of donors and acceptors are, as we see in the papers in the list of references, can be demonstrated even in one isolated paper, and one can realise (has realised) that the prime factor impacting on the overall outcomes as photocatalysts is rather than the excitonic effects in the actual systems. Thus I feel the impact of this paper in the field is a bit short and limited, and I am hesitating to recommend the paper in Nature Communications with extremely wide readership. Followings are some critical issues the authors should address in case of submission to the other journals or resubmission to this journal. In particular, the point two is substantial; I cannot recommend publication of this article unless addressing this point." Response: We much appreciate the comments from the reviewer. In response to the questions raised, we have made revisions and provided corresponding explanations below. 1) Thanks for your kind concern about the title. After re-assessing the content of the work and its title, we have now revised it to "Computation-Based Regulation of Excitonic Effects in Donor-Acceptor Covalent Organic Frameworks for Enhanced Photocatalysis", toning down the claim and making it easier for readers to understand the main purpose of the work. 2) Yes, there are indeed many factors influencing the performance of photocatalysis in D-A COFs. In this work, we aim to control the excitonic effect as the dominant factor, and fix all other possible variables in the photocatalysts investigated (the four D-A COFs). Previous reports (see refs: Appl. Catal. B 2020, 276, 119174;Chem. Soc. Rev. 2020, 49, 4135) have indicated that, the structure and energy levels of COFs are the primary factors affecting photocatalysis. It is apparent that the four representative D-A COFs share similar structures and energy levels (with a bandgap between 2.46 and 2.64 eV, and a LUMO level between -0.66 and -0.85 eV) (see SI: Figures S11-S15). In this case (for the photocatalysts with similar structures and energy levels), how would excitonic effects (or Eb) regulate photocatalysis? This is just what we would present to readers in this work.
In fact, in addition to the similar structures and energy levels of the four COFs, the following other minor factors possibly influencing photocatalytic performance have been considered: a) The static water contact angles of the four COFs, are in the range of 83°-96° (see SI: Figure S18), indicating their similar hydrophilicity and dispersibility. b) The sizes and loadings of Pt co-catalyst in the four COF photocatalysts are also similar (see SI: Figure S19, Table S6).
Related discussion on the excitonic effect as the prime/dominant factor impacting on the overall photocatalytic performance has been added in text and SI (see text: page 13, the 1 st paragraph, line 4-11; see SI: Figure S18-19).
3) The important features concerning the results reported in this work have been summarized as follows, which, we believe, are of great interest to general readers of Nature Commun.: a) In comparison with polymeric conductors, COFs facilitate precise fabrication and structural tailoring, giving the opportunity to accurately control over excitons and disclose the relationship between the strength of D-A interactions and excitonic effects. This work employs ideal photocatalysts (e.g., COFs) to investigate how D-A interaction strengths in COFs affect excitonic effects and the corresponding photocatalytic activity.
b) Though the formation of D-A structure is frequently mentioned in COF photocatalysis, whether the donor and acceptor monomers can truly give a D-A structure remains unclear in COFs. In this work, DFT calculations have been adopted to estimate the feasibility of D-A COF formation. Moreover, for a series of potential D-A COFs, the D-A interaction strength and exciton binding energy (Eb), as well as the corresponding photocatalytic activity trend, have been theoretically predicted.
c) According to the calculation results, seven D-A COFs can be formed out of ten possible combinations, in which the predicted D-A COF, i.e. TAPT-OMe-COF, featuring the strongest D-A strength yet the lowest Eb, would present the highest photocatalytic activity. This computational prediction makes it dispensable for a large number of experimental trials to the synthesis of COFs with ordinary performance, directly screening out the best-performance COF.
d) Four D-A COFs have been synthesized and their activity trend in the photocatalytic H2 production is in good agreement with the prediction by the calculation. Furthermore, the post-modification of the imine linkage to quinoline moiety in these D-A COFs enables them higher stability and much enhanced photocatalytic activity up to 7875 μmol h -1 g -1 due to the significantly reduced Eb. This work provides an effective strategy for improving photocatalytic activity, stability, and recyclability for imine COFs.
e) This work not only for the first time achieves the rational fabrication of high-performance COF photocatalysts directed by the pre-design and exciton binding energy calculation with computation guidance, but also provide a new research paradigm for advanced COF photocatalysis (from calculation to experiments, then further stability and performance optimization with computational elucidation).
Given the significance of our findings, we believe that this work will have a profound impact in the field of COF/polymer photocatalysis, and will encourage readers to pay greater attention to the impact of excitonic effects on photocatalysis, an area that has been long ignored.
Major comments: Comment 1: "In page 3 line 5, I don't recommend to use the term "oriented movement" in this context because both direct formation of exciton by electronic transition and indirect formation by charge transfer process are dominated by probability and its direction is also not determined, not oriented." Response: Many thanks for the very kind reminder. As per your suggestion, we have removed the term 'oriented movement'. Related revisions have been made in the text (see text: page 3, the 1 st paragraph, line 2 from the bottom). Fig 3? For me, they look all ellipsoid, it won't be in such a shape. If it is wrong, they are quite misleading. The authors already calculated in DFTlevel, so the difference in density can be accurately generated. (same for Fig7)?" Response: Thanks a lot for the professional comments. The form of Δρ (see text: Figure 3, the left-side image for each COF), as requested by the reviewer, presents the original charge density difference between the ground states (GS) and the excited states (ES). In fact, the form of centroids of charges (see text: Figure 3, the right-side image for each COF), which we assume to give a more visual and intuitive presentation of the density difference between the GS and ES, is just another form of representation for Δρ. The centroids of charge employs a method described for the computed charge difference in a previous theoretical study (J. Chem. Theory Comput. 2011, 7, 2498), which has been well accepted and cited over 800 times. This method has also been employed in many previous studies (e.g., Angew. Chem. Int. Ed. 2023, e202300256; Nat. Commun. 2021, 12, 320; Acc. Chem. Res. 2022Res. , 55, 2698Chem. Sci. 2020, 11, 3405) and it can be considered as a useful method to simplify charge density difference (Δρ).

Comment 2: "How did the authors generate the isosurface in
Specifically, the form of Δρ was generated using the following formula: The form of centroids of charges defines two centroids of charges ( and ) associated with the positive and negative density regions. Firstly, the root-mean-square deviations along the three axes ( , = , , ; = + −) are computed as Then the two centroids can then simply be defined as For more detailed information, please refer to the aforementioned JCTC (J. Chem. Theory Comput. 2011Comput. , 7, 2498. The structures of these centroids are obtained from Gaussian functions, where the values asymptotically approach zero from the centroid of the function, resembling the shape of an ellipsoid as concerned by the reviewer. The above-mentioned calculation method has been supplemented in the SI (see SI: page 7-8, S4 Supplementary Calculation Details).
Clearly, the form of Δρ appears to be less intuitive than centroids of charges, due to the intertwining of positive and negative charge densities, hindering the identification of actual donor and acceptor moieties as well as the comparison of the extent of charge transfer between different substituents. By contrast, the computed centroids, which involve the use of and functions, offer a clearer image, enabling better discernment of the spatial characteristics of charge transfer between the donor and acceptor. In addition, the distance of charge transfer (DCT), which is calculated from the total charge transfer length between the negative and positive barycenters, is better represented through this form. In response to your concerns, we have included the suggested form of charge density difference (Δρ) in the Figure 3 and 7 (see text: Figure 3 and 7a). Related revisions have been made in the text and SI (see text: page 8, the 1 st paragraph from the bottom, line 1-4 and Figure 3; page 17, Figure 7). Comment 3: "Please provide simulated XRD patterns for other samples including AA stacking and AB stacking in at least supporting information." Response: We are thankful for the constructive suggestion. The simulated XRD patterns including AA stacking and AB stacking for D-A COFs (see SI: Figure S6) and post-synthetic modified COFs (see SI: Figure S25) have been added in the SI.
Comment 2: "In the same context as my first comment, is there any advantage of the method used in this report to produce H2 over other methods? For example, with regards to complexity." Response: Thanks a lot for the valuable comments. In this work, compared to previous methods, the method employed in this work offers the following advantages: 1) From a research method perspective, this work employs DFT calculations to screen target photocatalysts using exciton binding energy as the primary index. Compared to traditional methods for photocatalysis research, this approach offers several advantages, including higher efficiency, reduced costs, and improved accuracy in predicting the performance of photocatalysts. 2) From a photocatalyst perspective, polymer semiconductors offer several advantages over traditional inorganic semiconductors, including not requiring metals, being low-toxic, inexpensive, and having adjustable band gaps. Especially for COFs, crystalline polymers with precise and tailorable structures, offer an advantageous platform for investigating the relationship between structure and photocatalytic activity through molecular-level structural regulation and hold great potential for advanced photocatalysis in hydrogen production. Comment 7: "Does the dihedral angle between the D and A components of the studied molecules have any correlation with the exciton binding energy?" Response: We are thankful for the interesting and nice direction. In our work, the studied molecules (D-A pairs) containing TAPTc part (TAPT-Cl, TAPT-H, TAPT-OCCH, TAPT-OH and TAPT-OMe) are planar (a dihedral angle of 0 degrees), while only TAPB-OMe has a dihedral angle of 43.3 degrees (see text: Figure  3). Given that TAPB-OMe exhibits the largest dihedral angle and exciton binding energy, it is reasonable to assume that a smaller dihedral angle would correspond to a decreased exciton binding energy. However, the limited number of examples of dihedral angles in this study makes it difficult to draw a reliable conclusion. Future work will further examine the relationship between the dihedral angle and exciton binding energy in COFs, but this would be a substantial undertaking and merit a manuscript in its own right.

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): The revised title of the manuscript becomes much better than the initial one, away from misleading of the readers. Stil the term of "computation" bothers me cause one can lead the choice of the chromophores among the building blocks of COFs by "thinking" without computations. It might be acceptable in the field of general chemistry.
The major changes made in this revised version of manuscript are in my comments 2 for the initial submission; the protocol to lead isosurface of figure 3. Indeed the use of differential charge density as discussed in J. Chem. Theory Comput. 2011 have given a better interpretation phenomenologically as the authors are referring the papers in ACIE, Nat. Commun. ACR, and Chem Sci. The last one is discussing on a bit shifted system from the ones in the present study, however minding us some new insights of experiments: electron paramagnetic resonance under photo-illumination. The system will provide a clear answer for the authors hypothesis of "computation guided design" of the system quantitatively by the clear correlation between the equilibrium yield of the primary intermediates and the choice of chromophores. It is, however, another story.
Overall I feel the manuscript is now ready for publication in Nature Communications.
Reviewer #2 (Remarks to the Author): The authors have implemented all suggestions and I believe the manuscript is in good shape for publication. For this reason, I congratulate the authors for the excellent work done.